German Pat. No. DT-AS 1,778,451 teaches the manufacture of multi-layer molded bodies with a foamed core and an unfoamed thermoplastic shell, wherein there is first injected into the mold a plug of a first charge of unfoamable thermoplastic material, which does not fill the mold. Then, before solidification of the mid-section of this plug of the first charge, there is injected into this mid-section a charge of thermoplastic material containing a foaming agent, whereby the material of the second charge presses the material of the first charge in every direction in the mold, so that it becomes fully filled. In this known method, the second charge is fed only when the input of the first charge has been interrupted or fully terminated. This frequently leads to undesirable markings at the surface of the finished molded bodies. Furthermore, particularly on molded bodies of complex shape or complex surface, the first-injected charge sometimes is not pressed uniformly against the walls of the mold by the subsequent second charge containing a raising agent, but is dispersed so far apart that it ruptures. As a result, the smooth surface of the shell is broken up and the molded body becomes useless.
German Pat. No. DT-OS 2,241,002 teaches further a method for the injection molding of plastic bodies with smooth surface and porous core, wherein subsequent to the injection of the portion part of the plastic that forms the smooth surface there is injected a plastic containing a foaming agent simultaneously with an additional portion of the plastic that forms the smooth surface. The plastic containing the foaming agent flows into the mold through a central nozzle, while the plastic which forms the smooth surface enters the mold through a ring nozzle, that encircles the central nozzle. In this method only a small amount of the plastic which forms the smooth surface can still reach (during the simultaneous injection phase of the two different plastics) that part of the mold wall which lies opposite the sprue. Therefore, at this site of the finished molded body (which most of the time forms the so-called "visual" side) the shell of plastic which forms the smooth surface is relatively thin and sometimes even tears. This shell is, however, unnecessarily thick at the site of the finished mold, which is adjacent to the sprue. It is not possible by this method to control in any economically-justifiable manner the thickness of the smooth-surfaced shell in the region of the visual side of the finished molded body. These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.
It is, therefore, an outstanding object of the invention to provide a method and apparatus for the intermittent manufacture of multi-layer molded bodies of thermoplastic material with a foamed core and an unfoamed shell, whereby it is assured (without increasing the amount of plastic that forms the unfoamed shell) that the unfoamed shell is approximately uniformly thick over the whole surface of the finished molded component and is, above all, absolutely compact.
With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.